How a Long-Term CO2-Induced Increase in Forest Productivity is Maintained on a Nitrogen-Impoverished Soil
Drake, J.E., Gallet-Budynek, A., Hofmockel, K.S., Bernhardt, E.S., Billings, S.A., Jackson, R.B., Johnsen, K.S., Lichter, J., McCarthy, H.R., McCormack, M.L., Moore, D.J.P., Oren, R., Palmroth, S., Phillips, R.P., Pippen, J.S., Pritchard, S.G., Treseder, K.K., Schlesinger, W.H., DeLucia, E.H. and Finzi, A.C. 2011. Increases in the flux of carbon belowground stimulate nitrogen uptake and sustain the long-term enhancement of forest productivity under elevated CO2. Ecology Letters 14: 349-357.
Based on multiple measurements of numerous plant and soil properties and processes that were made over a twelve-year period at the Duke Forest free-air CO2 enrichment (FACE) facility -- where atmospheric CO2 concentrations were maintained approximately 200 ppm above ambient throughout the forest volumes of half of the facility's six 30-meter-diameter plots -- the twenty researchers report that the CO2-induced enhanced rates of net primary production are sustained by a carbon-cascade through the root-microbe-soil system, whereby "increases in the flux of carbon belowground under elevated CO2 stimulated microbial activity" that in turn "accelerated the rate of soil organic matter decomposition and stimulated tree uptake of nitrogen bound to this soil organic matter," which process "set into motion a positive feedback maintaining greater carbon gain under elevated CO2 as a result of increases in canopy nitrogen content and higher photosynthetic nitrogen-use efficiency," the consequence of which chain of events was "the dominance of carbon storage in tree biomass but the preclusion of a large carbon sink in the soil."
In discussing this chain of events, Drake et al. say "the long-term increase in forest productivity under elevated CO2 at the Duke FACE site appears to be maintained by a belowground exchange of tree carbon for soil nitrogen, with the quantity of carbon allocated belowground set by the availability of nitrogen in the soil and the demand for nitrogen to meet growth requirements." In fact, they write that "all of the belowground carbon fluxes thought to increase decomposition rates increased under elevated CO2, including root production and mortality (Pritchard et al., 2008a), root exudation (Phillips et al., 2011), fungal rhizomorph production (Pritchard et al., 2008b) and allocation of carbon to mycorrhizal fungi (Garcia et al., 2008)." And, therefore, they conclude that "the preponderance of the evidence points to increased decomposition [of organic matter] in surface soils as the primary source of additional nitrogen taken up by the trees growing under elevated CO2."
Last of all, Drake and colleagues additionally note that ectomycorrhizal fungi (EMF) colonize the roots of the plant communities at the Duke and Florida sites, whereas arbuscular mycorrhizal fungi (AMF) colonize the roots of two other highly-studied plant communities where a partial down regulation of net primary production under elevated CO2 has been observed (Reich et al., 2006; Norby et al., 2010). And they thus speculate that "fungal community composition may mediate positive- vs. negative feedback effects of elevated CO2 on net primary production," which is an hypothesis that clearly warrants further scrutiny.
Garcia, M.O., Ovaspyan, T., Greas, M. and Treseder, K.K. 2008. Mycorrhizal dynamics under elevated CO2 and nitrogen fertilization in a warm temperate forest. Plant and Soil 303: 301-310.
Langley, J.A., McKinley, D.C., Wolf, A.A., Hungate, B.A., Drake, B.G. and Megonigal, J.P. 2009. Priming depletes soil carbon and releases nitrogen in a scrub-oak ecosystem exposed to elevated CO2. Soil Biology and Biochemistry 41: 54-60.
McCarthy, H,.R., Oren, R., Johnsen, K.H., Gallet-Budynek, A., Pritchard, S.G., Cook, C.W., LaDeau, S.L., Jackson, R.B. and Finzi, A.C. 2010. Re-assessment of plant carbon dynamics at the Duke free-air CO2 enrichment site: interactions of atmospheric [CO2] with nitrogen and water availability over stand development. New Phytologist 185: 514-528.
Norby, R.J., Warren, J.M., Iversen, C.M., Medlyn, B.E. and McMurtrie, R.E. 2010. CO2 enhancement of forest productivity constrained by limited nitrogen availability. Proceedings of the National Academy of Sciences USA 107: 19,368-19,373.
Phillips, R.P., Finzi, A.C. and Bernhardt, E.S. 2011. Enhanced root exudation induces microbial feedbacks to N cycling in a pine forest under long-term CO2 fumigation. Ecology Letters 14: 187-194.
Pritchard, S.G., Strand, A.E., McCormack, M.L., Davis, M.A., Finzi, A.C., Jackson, R.B., Matamala, R., Rogers, H.H. and Oren, R. 2008a. Fine root dynamics in a loblolly pine forest are influenced by free-air-CO2-enrichment: a six-year-minirhizotron study. Global Change Biology 14: 588-602.
Pritchard, S.G., Strand, A.E., McCormack, M.L., Davis, M.A. and Oren, R. 2008b. Mycorrhizal and rhizomorph dynamics in a loblolly pine forest during 5 years of free-air-CO2-enrichment. Global Change Biology 14: 1252-1264.
Reich, P.B., Hobbie, S.E., Lee, T., Ellsworth, D.S., West, J.B., Tilman, D., Knops, J.M.H., Naeem, S. and Trost, J. 2006. Nitrogen limitation constrains sustainability of ecosystem response to CO2. Nature 440: 922-925.